A survey of the health status of parents and grandparents of patients with homocystinuria due to cystathionine synthase deficiency has been completed. The aim was to determine whether the mild homocyst(e)inemia present in heterozygotes for this recessively inherited condition are at increased risk for atherosclerotic diseases: heart attacks and strokes. The information upon these relatives was compared to comparable information upon relatives of patients with either impaired phenylalanine metabolism or new mutation achondroplastic dwarfism. Age-stratified comparison of the incidence of heart attacks and/or strokes failed to reveal significant increases consistently present in relatives of homo-cystinurics. Life table analyses of cumulative incidence of fatal heart attacks as a function of age likewise revealed no striking differences. No more than 5 percent of heterozygotes will have a coronary attack by age 50. These results indicate that mild homocyst(e)inemia is not an important contributory factor to atherosclerotic disease in the general population and provide no support for current hypotheses which postulate such an association.